1. Field of the Invention
The present invention relates to a wafer and a wafer cutting and dividing method.
2. Description of Related Art
A dicing (laser dicing) technique, which uses a laser beam to cut and divide a wafer-like workpiece into individual chips, has been under development.
For example, as recited in Japanese Patent No. 3408805 that corresponds to U.S. Pat. No. 6,992,026B2, US2005/0173387A1, US2005/0181581A1, US2005/0184037A1, US2005/0189330A1, US2005/0194364A1, US2006/0040473A1 and US2006/0160331A1, it has bee proposed that the laser beam is irradiated on the wafer-like workpiece in such a manner that a focal point of the laser beam is placed in the interior of the wafer-like workpiece to form modified areas (modified areas including crack areas, modified areas including fused areas, modified areas including areas where a refractive index changes) through multiphoton absorption from the laser beam. A cutting start area is formed by the modified areas in the wafer-like workpiece along a predetermined cutting line of the wafer-like workpiece at a predetermined depth from a laser beam incident surface of the wafer-like workpiece. The cutting of the wafer-like workpiece is initiated along the cutting start areas to cut and divide the wafer-like workpiece.
Furthermore, as recited in Japanese Unexamined Patent Publication No. 2002-205180 that corresponds to U.S. Pat. No. 6,992,026B2, US2005/0173387A1, US2005/0181581A1, US2005/0184037A1, US2005/0189330A1, US2005/0194364A1, US2006/0040473A1 and US2006/0160331A1, it has been also proposed that the laser beam is irradiated on the wafer-like workpiece to form the modified areas in the interior of the wafer-like workpiece along the predetermined cutting line. However, in this instance, a position of a focal point of the laser beam in an incident direction of the laser beam to the wafer-like workpiece is changed in the interior of the wafer-like workpiece from one to another to form multiple rows of the modified areas in the incident direction of the laser beam.
According to this Japanese Patent Publication No. 2002-205180, the multiple rows of modified areas are formed in the wafer-like workpiece in the incident direction of the laser beam. Thus, the number of cutting start areas is also increased, and thereby the wafer-like workpiece having a relatively large thickness can be easily cut along the cutting start areas.
Furthermore, as recited in Japanese Unexamined Patent Publication No. 2005-1001, which corresponds to US2006/0011593A1 and US2005/0202596A1, an expansible film may be applied to one of opposed surfaces of a planar workpiece, which includes a substrate, and a laser beam is irradiated into an interior of the substrate through the other one of the opposed surfaces of the workpiece to place a focal point of the laser beam in the interior of the workpiece, so that modified areas (fused areas) are formed by multiphoton absorption from the laser beam. The thus formed modified areas may be used to form cutting start areas in the predetermined depth of the workpiece, which is spaced by a predetermined distance from the laser beam incident surface of the workpiece, along the predetermined cutting line of the workpiece. Then, the film may be expanded to cut the workpiece into multiple pieces in such a manner that the cutting is initiated in the cutting start areas.
According to the technique recited in Japanese Unexamined Patent Publication No. 2005-1001, the film is expanded after the formation of the cutting start areas in the interior of the substrate, so that the stretching stress can be appropriately applied to the cutting start areas to start the cutting initially from the cutting start areas, and thereby the substrate can be relatively accurately cut and divided into the pieces with a relatively small force.
In the recent years, the multi-layering technique of the semiconductor substrate is progressed, and the laser dicing technique recited in, for example, Japanese Patent No. 3408805, Japanese Unexamined Patent Publication No. 2002-205180 or Japanese Unexamined Patent Publication No. 2005-1001 is applied to a wafer (a semiconductor wafer), which is used in manufacturing of a semiconductor substrate having the multi-layers, to cut and divide the wafer into individual chips (semiconductor chips).
The multi-layering technique of the semiconductor substrate may include a bonding technique, a Separation by Implanted Oxygen (SIMOX) technique, a silicon on insulator (SOI) technique, a crystal growth technique for growing a III-V family chemical compound semiconductor layer on a substrate (e.g., sapphire) or a bonding technique for bonding a silicon substrate and a glass substrate together through use of anodic bonding.
FIG. 14 is a descriptive view, which indicates a way of forming modified areas by irradiating a laser beam on a wafer 50 having a bonded SOI structure according to a previously proposed technique and which schematically shows a longitudinal cross section of the wafer 50.
The wafer 50, which has the bonded SOI structure, includes a substrate Si (single crystal silicon) layer 51, a buried oxide (BOX) layer 52 and an SOI (single crystal silicon) layer 53 in this order from the bottom side to the top side thereof. Thus, the wafer 50 has the SOI structure, in which the single crystal silicon layer 53 is formed on the buried oxide layer 52 that is an insulation layer.
Here, the wafer 50, which has the bonded SOI structure, may be produced by bonding two wafers, each of which has a bonding surface (a mirror surface) that is thermally oxidized to form an oxide film thereon through the oxide films. Then, one of the two wafers is polished to a desired thickness. Here, the polished wafer becomes the SOI (single crystal silicon) layer 53, and the unpolished wafer becomes the substrate Si (single crystal silicon) layer 51, and the oxide films become the buried oxide layer 52.
A dicing film (a dicing sheet, a dicing tape, an expanded tape) 54 is bonded to the back surface (the lower surface of the single crystal silicon layer 5.1) 50a of the wafer 50.
The dicing film 54 is made of an expansible plastic film, which expands when the film is heated or when a force is applied to the film in an expansion direction. The dicing film 54 is bonded to the entire back surface of the wafer 50 through a bonding agent (not shown).
A laser processing machine (not shown) includes a laser beams source (not shown) for outputting a laser beam L and a converging lens CV. In a state where an optical axial OA of the laser beam L is placed perpendicular to the surface 50b of the wafer 50, the laser beam L is irradiated on the surface (the laser beam incident surface) 50b of the wafer 50 through the converging lens CV such that a focal point (light converging point) P of the laser beam L is placed at a predetermined point in the interior of the wafer 50. Therefore, a modified area (a modified layer) is formed at the focal point P in the interior of the wafer 50.
The laser beam L may be a laser beam that has a wavelength of 1064 nm, which is in an infrared wavelength range.
Here, the modified areas R include fused areas, which are generated mainly through the multiphoton absorption caused by the irradiation of the laser beam L.
Specifically, a portion of the wafer 50 at the focal point P of the laser beam L in the interior of the wafer 50 is locally heated through the multiphoton absorption from the laser beam L, so that the portion of the wafer 50 is melted once and is then resolidified. As described above, the portion of the wafer 50, which is melted and is then resolidified, becomes the modified area R.
That is, the fused area refers to an area, which has undergone the phase change, or an area, which has a changed crystal structure. In other words, the fused area refers to an area, in which the single crystal silicon is changed to the amorphous silicon, an area, in which the single crystal silicon is changed to the polycrystal silicon, or an area, in which the single crystal silicon is changed into a structure having the amorphous silicon and the polycrystal silicon. The wafer 50 is a bulk silicon wafer, so that the fused area is mainly made of the polycrystal silicon.
The fused area is formed mainly by the multiphoton absorption rather than simple absorption of the laser beam L in the interior of the wafer 50 (i.e., rather than the heating by the normal laser beam).
Thus, the laser beam L is not substantially absorbed in the area other the focal point P of the laser beam L in the interior of the wafer 50, and the top surface 50b of the wafer 50 is not melted.
The pulsed laser beam L is applied on the wafer 50 by the laser processing machine such that the laser beam L is scanned, i.e., run over the wafer 50 while the depth position of the focal point of the laser beam L in the interior of the wafer 50 is kept constant. In this way, the focal point P is moved along a predetermined straight cutting line (i.e., in a direction of an arrow α).
FIG. 14 shows a state where the laser beam L is scanned in a direction parallel to a plane of the drawing.
Here, it should be noted that the irradiating position of the laser beam L from the laser processing machine may be fixed without scanning the laser beam L by the laser processing machine. In this state, a table (not shown), which supports the wafer 50, may be moved in a direction perpendicular to an impinging direction of the laser beam L, i.e., an optical axis of the laser beam L (the incident direction of the laser beam L to the top surface 50b of the wafer 50).
Specifically, the focal point P of the laser beam L may be relatively moved with respect to the wafer 50 along the predetermined cutting line of the wafer 50 either by scanning the laser beam L or moving the wafer 50.
As described above, in the state where the depth position of the focal point P of the laser beam L in the interior of the wafer 50 is kept constant, when the pulsed laser beam L is irradiated in such a manner that the focal point P of the laser beam L is relatively moved with respect to the wafer 50, the multiple modified areas R (a modified area group including the multiple modified areas R) are formed at constant intervals d in a direction parallel to the top surface 50b and the back surface 50a of the wafer 50 at a fixed depth from the top surface 50b of the wafer 50 (i.e., a position that is spaced by a predetermined distance from the laser beam incident surface 50b of the wafer 50, on which the laser beam L is impinged), so that a layer of a modified area group Ga-Gc is formed.
Here, the depth of the focal pint P of the laser beam L in the interior of the wafer 50 is defined as a distance from the top surface (the laser beam incident surface) 50b of the wafer 50.
Also, the interval d of the modified areas R is defined as a center-to-center distance between a left-to-right center of one of corresponding adjacent two modified areas R and a left-to-right center of the other one of the two modified areas R in a left-to-right direction of FIG. 14 (in a direction parallel to the top surface 50b and the back surface 50a of the wafer 50).
Here, the interval d of the modified areas R of each modified area group Ga-Gc is set to be a value (d=s/f), which is obtained by dividing the relative moving speed s of the focal point P of the laser beam L with respect to the wafer 50 (the scanning speed of the laser beam L or the moving speed of the wafer 10) by the pulse oscillation frequency (a pulse repetition frequency) f of the pulsed laser beam L.
That is, in the case where the relative moving speed s of the focal point P is constant, the interval d of the modified areas R gets larger as the pulse oscillation frequency f of the laser beam L gets lower. Furthermore, in the case where the pulse oscillation frequency f of the laser beam L is constant, the interval d of the modified areas R gets larger as the relative moving speed s of the focal point P gets higher.
When the depth position of the focal point P in the interior of the wafer 50 is changed stepwise, multiple layers of modified area groups Ga-Gc are formed by the laser processing machine along the predetermined cutting line of the wafer 50 at constant intervals in a depth direction of the wafer 50 (i.e., the thickness direction of the wafer 50, the cross sectional direction of the wafer 50, the perpendicular direction that is perpendicular to the top and back surfaces 50b, 50a of the wafer 50, the top-to-bottom direction of the wafer 50), which is perpendicular to and is directed from the top surface 50b of the wafer 50.
The position (the depth position) of the focal point P of the laser beam L in the incident direction of the laser beam L on the wafer 50 (the depth direction of the wafer 50) is changed multiple times, so that the corresponding modified areas R of the layers of the modified area groups Ga-Gc are aligned in the incident direction of the laser beam while a desired interval is provided between each corresponding two modified areas R in the incident direction of the laser beam.
For example, the first layer (the lowermost layer) of the modified area group Ga is formed by relatively moving the focal point P in a state where the depth position of the focal point P is set adjacent to the back surface 50a of the wafer 50. Then, the second layer (the intermediate layer) of the modified area group Gb is formed by relatively moving the focal point P in a state where the depth position of the focal point P is set generally at a half point between the top surface 50b and the back surface 50a of the wafer 50. Thereafter, the third layer (the uppermost layer) of the modified area group Gc is formed by relatively moving the focal point P in a state where the depth position of the focal point P is set adjacent to the top surface 50b of the wafer 50.
In the case of FIG. 14, although the three layers of the modified area groups Ga-Gc are provided, the number of the layers of the modified area groups is not limited to three and may be set to two or less or four or more.
Here, in the case of the layers of the modified area groups Ga-Gc, it is desirable that the layers of the modified area groups Ga-Gc are formed one after anther from the farthest layer to the closest layer (in the order of Ga, Gb and Gc) with respect to the top surface (the laser beam incident surface) 50b of the wafer 50, on which the laser beam L impinges.
For example, in a case where the farthest layer of the modified area group Ga is formed after the formation of the closest layer of the modified area group Gc, the laser beam L applied to form the modified area group Ga is scattered by the previously formed modified area group Gc. Thus, the size of the modified area R of the modified area group Ga varies from one modified area R to another modified area R, so that the modified area Ga cannot be formed uniformly.
However, when the modified area groups Ga-Gc are formed one after anther from the farthest layer of the modified area group Ga to the closest layer of the modified area group Gc, it is possible to form a new modified area R with the focal point P of the laser beam L while no modified area R is yet formed between the incident surface 50b and the current focal point P of the laser beam L. Therefore, at this time, the laser beam L is not scattered by the previously formed modified areas R, and thereby the multiple layers of the modified area groups Ga-Gc can be uniformly formed.
However, the forming sequence of the layers of the modified area groups Ga-Gc are not limited to this and may be appropriately experimentally set through actual experiments since in some cases, generally uniform modified area groups can be possibly obtained even when the layers of the modified area groups Ga-Gc are formed one after another from the closest layer of the modified area group Gc to the farthest layer of the modified area group Ga (in the order of Gc, Gb and Ga) with respect to the top surface 50b of the wafer 50, or even when the layers of the modified area groups Ga-Gc are formed at a random layer forming sequence.
The layers of the modified area groups Ga-Gc can be formed by changing the depth position of the focal point P in the interior of the wafer 50 through, for example, any one of the following methods (I)-(III).
(I) In one method, a head (a laser head), which includes the laser beam source for outputting the laser beam L and the converging lens CV, may be moved in the direction perpendicular to the top surface 50b and the back surface 50a of the wafer 50.
(II) In another method, the table, which supports the wafer 50, may be moved in the direction perpendicular to the top surface 50b and the back surface 50a of the wafer 50.
(III) In another method, the above two methods (I) and (II) may be combined to vertically move both of the head and the table in opposite directions. According to the method (III), the time required to form the layers of the modified area groups Ga-Gc can be reduced in comparison to the methods (I) and (II).
As described above, the multiple layers of the modified area groups Ga-Gc are formed in the interior of the wafer 50, and then the dicing film 54 is stretched in the horizontal direction with respect to the respective predetermined cutting line to apply the stretching stress to the modified area groups Ga-Gc.
In the case of FIG. 14, the dicing film 54 is stretched in the direction perpendicular to the plane of FIG. 14.
Thus, the shearing stress is generated in the interior of the wafer 50. As a result, a crack (break) is generated in the depth direction of the wafer 50 from the lowermost layer of the modified area group Ga, which is closest to the dicing film 54 and serves as a crack start point. Then, another crack is generated in the depth direction of the wafer 50 from the intermediate layer of the modified area group Gb, which serves as a crack start point. Thereafter, the crack is generated in the depth direction of the wafer 50 from the uppermost layer of the modified area group Gc, which serves as a crack start point. These cracks grow further and are connected to each other. When the grown cracks reach the top and back surfaces 50b, 50a of the wafer 50, the wafer 50 is cut and is divided.
Here, the modified area groups Ga-Gc are formed along the predetermined cutting line. Thus, when the stretching stress is appropriately applied to each modified area group Ga-Gc by stretching the dicing film 54, the wafer 50 can be relatively accurately cut and divided with a relatively small force without generating unnecessary cracks in the wafer 50 through the cracking started at the respective modified areas R in the layers of the modified area groups Ga-Gc.
In the top surface 50b of the wafer 50, which has a generally circular disc shape, chips are regularly arranged in a grid pattern. Each of the predetermined cutting lines is provided between the chips. That is, multiple predetermined cutting lines are arranged to form the grid pattern on the top surface 50b of the wafer 50.
Thus, after the formation of the modified area groups Ga-Gc along the predetermined cutting lines, the dicing film 54 is stretched. Therefore, the wafer 50 is cut and divided into the chips.
In the previously proposed technique shown in FIG. 14 and the prior arts recited in Japanese Patent No. 3408805, Japanese Unexamined Patent Publication No. 2002-205180 and Japanese Unexamined Patent Publication No. 2005-1001, each of the relative moving speed s of the focal point P and the pulse oscillation frequency f is set to a corresponding constant value, so that each interval d (=s/f) of the modified areas R in each modified area group Ga-Gc is made constant.
Therefore, in the case where the wafer 50 is the bulk silicon wafer or the bulk silicon wafer having an oxide film on its surface, it is possible to reliably form the normal modified areas R in each of the layers of the modified area groups Ga-Gc, which include the lowermost layer to the uppermost layer.
However, in the case of the wafer 50, which has the bonded SOI structure, although it is possible to form the normal modified areas R in the uppermost layers of the modified area group Gc, it is difficult to form the normal modified areas R in the intermediate layer of the modified area group Gb and in the lowermost layer of the modified area group Ga.
As described above, It is difficult to form the normal modified areas R at the deep part (the deep location), which is deep from the top surface (the laser beam incident surface) 50b of the wafer 50, due to the following reason.
That is, in the wafer 50, which has the bonded SOI structure, due to a variation in the optical characteristics of each layer 51-53, the refractive index of the laser beam L varies according to the layer thickness and material of each layer 51-53.
Thus, at a boundary surface between the layer 51 and the layer 52 or between the layer 52 and the layer 53, a portion of the laser beam L is reflected. This reflected laser beam interferes with the newly impinging laser beam to cause cancellation between the reflected laser beam and the newly impinging laser beam. Therefore, the energy of the laser beam L is reduced. Furthermore, the laser beam L, which has entered into the wafer 50, is absorbed in the interior of the wafer 50. Thus, as the depth from the top surface (the laser beam incident surface) 50b of the wafer 50 gets deeper, the energy of the laser beam L is more reduced.
As a result, at the deep part of the wafer 50, the energy of the laser beam L, which is required to cause the multiphoton absorption, becomes insufficient, so that the formation of the modified areas R, which include the fused areas, becomes impossible.
FIG. 15 schematically shows the longitudinal cross section of the wafer 50, in which the layers of the modified area groups are formed.
In the case of FIG. 15, the layers of the modified area groups are formed in the wafer 50 having the total thickness of 650 μm under the processing condition where each of the relative moving speed s of the focal point P of the laser beam L and the pulse oscillation frequency f of the laser beam L is set to be constant.
In the case of FIG. 15, the normal modified areas R are formed in each of the layers of the modified area groups in a portion 50c, which ranges from the top surface (the laser beam incident surface) 50b of the wafer 50 to the depth of 478 μm. However, in a deeper portion 50d, which is deeper than the depth of 478 μm, the modified areas R are not formed.
In the wafer 50, which does not have the normal modified areas R all the way from the lowermost layer of the modified area group Ga to the uppermost layer of the modified area group Gc, unnecessary cracks are easily formed at the time of cutting and dividing the wafer 50. Thus, it is difficult to relatively accurately cut and divide the wafer 50 along the predetermined cutting lines. Therefore, the yield and the quality of the chips made from the wafer 50 are deteriorated.
In the recent years, as recited in Japanese Patent No. 3408805, Japanese Unexamined Patent Publication No. 2002-205180 and Japanese Unexamined Patent Publication No. 2005-1001, it has been attempted to cut thicker wafers with the laser dicing technique.
However, in the technique recited in Japanese Patent No. 3408805, Japanese Unexamined Patent Publication No. 2002-205180 and Japanese Unexamined Patent Publication No. 2005-1001, it is required to provide the greater number of layers of the modified areas and to reduce the intervals d of the modified areas in all of the layers of the modified area groups, which include the lowermost layer to the uppermost layer.
Therefore, the relatively long time period is required to form the layers of the modified area groups, and thereby the throughput (the productivity per unit time) is deteriorated. As a result, the above technique is not suitable for the mass production.
Furthermore, the output power W of the laser beam L needs to be increased to form the normal modified areas R in each of the layers of the modified area groups. Thus, the power consumption of the laser processing machine, which generates the laser beam L, is disadvantageously increased to cause a disadvantageous increase in the manufacturing cost at the time of cutting and dividing the wafer.
Furthermore, in the recent years, it has been demanded to reliably form the normal modified areas through use of the laser dicing technique to improve the accuracy at the time of cutting other types of wafers made of the other type of material (e.g., a material that includes glass) other than the wafer made of the semiconductor material used in the manufacturing of the semiconductor substrate.